Information can be sent over a communication medium such as a wire using tones of different amplitude, phase and/or frequency. For example, information can be transmitted using a dual-tone signal comprising a first tone having a first frequency and a second tone having a second frequency. To accommodate transmitters and receivers that use asynchronous clocks, data is encoded in a dual-tone signal based on the ratio of the two frequencies rather than based on the actual first and second frequencies. To receive data encoded or represented by a dual-tone signal, a receiver may estimate the first and second frequencies, compute a ratio of the estimated frequencies, and decode the ratio to determine the transmitted data.